Debonding damage

If the interfacial bonding strength of the composite material is less than the cohesive strength of the matrix resin and the strength of the filler, a stress concentration near the filler can easily occur when composite materials are subjected to shear or tensile stress. Debonding damage will occur near the interface if the interfacial bonding is weak. 

Figure I represents a two-dimensional damage distribution of an impact in a 0/90° CFRP laminate of 3 mm thickness. Unlike in ultrasonic testing, which is usually the standard method for this problem, there is no shadowing effect on the successive layers by delamination echos. With the method of X-ray refraction the exact concentration of debonded fibers can be calculated for each position averaged over the wall thickness. Additionally the refraction allows the selection of the fiber orientation. The presented X-ray refraction topograph detects selectively debonded fibers of the 90° direction.

Fig. 2 X-ray refraction topographs of a series of /OyPOj/s samples of different impact energies. The total damage of the laminates is characterized by addition of all debonded layers of0° and 90° fiber direction.

Artefacts Damaged piece of GFRP to show opacity caused by debonding. 

In most GRPs debonding can occur after even a small number of cycles, even at modest levels. If the material is translucent, the buildup of fatigue damage can be observed. The first signs (for example, with glass-fiber TS polyester) are that the material becomes opaque each time the load is applied. Subsequently, the opacity becomes permanent and more pronounced, as can occur in... 

The analytical solutions derived in Sections 4.3 and 4.4 for the stress distributions in the monotonic fiber pull-out and fiber push-out loadings are further extended to cyclic loading (Zhou et al., 1993) and the progressive damage processes of the interface are characterized. It is assumed that the cyclic fatigue of uniform stress amplitude causes the frictional properties at the debonded interface to degrade... 

In short fiber composites, energy absorption mechanisms, such as interfacial debonding and matrix cracking, most often occur at the fiber ends (Curtis et al., 1978). The damage model proposed by Bader et al. (1979) assumes that short fiber composites fail over a critical cross-section which has been weakened by the accumulation of cracks, since the short fibers bridging this critical zone are unable to carry the load. In fatigue loading, sudden fracture takes place as a direct result from the far-field effect of the composite, rather than due to the near field of the crack tip... 

The earliest works of trying to model different length scales of damage in composites were probably those of Halpin [235, 236] and Hahn and Tsai [237]. In these models, they tried to deal with polymer cracking, fiber breakage, and interface debonding between the fiber and polymer matrix, and delamination between ply layers. Each of these different failure modes was represented by a length scale failure criterion formulated within a continuum. As such, this was an early form of a hierarchical multiscale method. Later, Halpin and Kardos [238] described the relations of the Halpin-Tsai equations with that of self-consistent methods and the micromechanics of Hill [29],... 

Three underlying mechanisms are responsible for the nonlinearity.17,18 (1) Frictional dissipation occurs at the fiber/matrix interfaces, whereupon the sliding resistance of debonded interfaces, r, becomes a key parameter. Control of t is critical. This behavior is dominated by the fiber coating, as well as the fiber morphology.19,20 By varying r, the prevalent damage mechanism and the resultant non-linearity can be dramatically modified. (2) The matrix cracks... 

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